Removed accessor generic functions for timestamps.

The plan is to detect obsolete objects by using the unique class
number rather than using timestamps.  The unique number of the class
will be stored in the instance just like the timestamp used to be, but
it will not be available as an explicit slot, so no replacement
accessors are required.
Removed file containing obsolete code.
Added writer (SETF UNIQUE-NUMBER).

The unique number of a class will change when the class is
reinitialized, so we need a writer for the corresponding slot.
Store the unique number of the class in the instance.

Instead of storing a new timestamp in the instance as before, we now
store the unique number of the class instead.  We can therefore remove
the entire timestamp mechanism as well.
Removed timestamp as a slot in STANDARD-OBJECT.

We no longer use timestamps, and instead we store the unique number of
the class in the instance.  However, the class unique number will
always be the first element of the contents vector so we do not need
to turn it into an ordinary slot.  For that reason, the slot
specification is removed entirely from the DEFCLASS form.